Recently, introduction of touch panel systems to various kinds of electronic devices has been growing rapidly. For example, the touch panel systems are introduced to portable information devices such as smartphones and automatic vending machines such as automatic ticket machines.
The touch panel system is typically configured to include (i) a display device and (ii) a touch panel stacked on an upper side (front surface) of the display device. Therefore, a sensor provided on the touch panel is likely to be affected not only by a noise such as a clock generated in the display device but also by other noises coming from the outside. Such the noises lead to impairment in detection sensitivity for a touch operation.
Patent Literature 1 describes a touch panel system (coordinates input device) including a countermeasure against such noises. The touch panel system of Patent Literature 1 includes a noise processing section for removing a noise. FIG. 19 is a block diagram illustrating a noise processing section 100 included in the touch panel system of Patent Literature 1. As shown in FIG. 19, the noise processing section 100 includes a filter section 101, a logical inversion section 102, and an adding section 103. The filter section 101 receives an output signal (analog signal) from a sensor provided in a touch panel (not illustrated). The filter section 101 extracts, as a noise signal, an AC signal component included in the input signal. The logical inversion section 102 inverts by 180° the phase of the noise signal thus extracted. The adding section 103 adds, to the input signal which is supplied to the filter section 101 and which includes the noise signal, the noise signal whose phase has been inverted by 180°.
Thus, according to the touch panel system of Patent Literature 1, the noise signal extracted by the filter section 101 is inverted, and the signal thus inverted is added to the input signal (analog signal) supplied from the sensor. Namely, to the noise component included in the input signal supplied from the sensor, such a signal is added which has the same level as the noise component and whose phase has been inverted. This cancels the noise superimposed on the input signal supplied from the sensor. This makes it possible to reduce effects given by the noise included in the input signal supplied from the sensor.
On the other hand, as shown in Patent Literature 2, a conventional touch panel system configured to detect how capacitance values are distributed has been trying to carry out recognition of a finger and part of a hand which are in contact with a touch panel by means of signal processing. For example, assume that a hand holding a stylus is in contact with a touch panel. A signal based on how the touch panel is touched significantly changes over time according to action of moving the stylus etc.
(a) to (d) of FIG. 34 are views for describing touch signals observed when a hand is placed on a touch panel. As shown in (a) of FIG. 34, a region where the hand is placed on the touch panel is small at first, i.e., the region is a region 216a. Then, as shown in (b) of FIG. 34, the region where the hand is placed on the touch panel expands over time to a region 216b. Next, as shown in (c) of FIG. 34, the region where the hand is placed on the touch panel further expands over time, and a tip of a stylus held in the hand makes contact with the touch panel. This causes a stylus input region 218 to appear. After that, as shown in (d) of FIG. 34, the region where the hand is in contact with the touch panel changes from the region 216c to a region 216d, and a region 217 where a finger is in contact with the touch panel appears.